Certain applications of polyurethanes require both low hardness and highly consistent surface characteristics. A particular example of this is in the use of polyurethanes as roller material for electrophotography, such as developer rollers, for which inconsistent or contaminated surfaces result in poor print quality.
In electrophotography the developer roll functions to develop a layer of toner onto a charged photoconductor drum. The toner is metered onto the surface of the developer roll via a doctor blade. The electrical properties of the developer roll assist in the electrostatic transfer of charged toner from the roller surface to the photoconductor drum. The use of a two layer, “coated” roll will develop a fixed quantity of toner per volt of development bias that is determined by the dielectric thickness of the photoconductor, the toner, and the developer roll. This development characteristic is independent of process speed, within limits.
In contrast, a solid roll of a single resistivity develops a quantity of toner based on the dielectric constants of the photoconductor and the toner, and the resistance of the roll in the photoconductor nip, which is dependent on process speed. In addition, a two-layer roll has a longer time constant than a single layer roll. As a result, the two-layer rolls have a higher effective development surface at the entry to the photoconductor nip. This improves the single pel dot print performance of the roll. Therefore, the print performance of a two-layer roll is superior to that of a single-layer roll across a wide process speed range and is less sensitive to varying environmental conditions. The desired electrical properties of a two-layer roll are; a core resistivity less than 1×109 ohm-cm, preferably less than 3×108 ohm-cm, at 22° C. and 50% relative humidity (RH), a coating resistivity of 5×109 to 2×1012 ohm-cm, preferably 1×1011 to 5×1011 ohm-cm, at 22° C. and 50% RH and a coating thickness of approximately 30-200 μm, preferably around 100 μm. The time constant should be 5-2000 milliseconds, preferably about 100 milliseconds, at 22° C. and 50% RH.
As is now well established in the prior art, a resistive surface layer can be produced on a cast urethane roll by baking in air at an elevated temperature. The oxidation of polybutadiene, in the presence of certain conductive metal salts, produces a highly resistive layer at the surface of the roll. The thickness and resistivity of this layer can be adjusted by varying the polybutadiene and metal salt concentrations in the urethane, the baking time, the baking temperature, and the concentration of oxygen.
A softer, more compliant roller is less susceptible to showing non-uniformities due to a more uniform nip at the interfaces of the developer roll with both the doctor blade and the photoconductor drum. Typically, a plasticizer is added to reduce roller hardness. However, such a material is not reacted into the polymer matrix and is free to migrate throughout the rubber roller including to the surface. At the roller surface such additives will interact with the toner causing it to coalesce in the nip area of the developer roll and the photoconductor drum. Reactive high molecular weight polyols have been demonstrated to lower the hardness of polyurethanes, however, conventional α,ω-telechelic curatives in which the OH functional groups are located at opposite ends of the polymer chain afford materials of increased compression set, severely limiting their utility in roller applications. It has been demonstrated that addition of high molecular weight graft polyols, which react into the polymer backbone, decreases the hardness of the roller without compromising the low compression set characteristics.
As the number and variety of polyurethane curatives in each roll formulation increase, however, so do issues of curative compatibility and immiscibility, which often result in a phase separation of the components within the cured polyurethane matrix. This separation causes domains or defects within the roll and on the roll surface termed “potholes.”